Improved Design and Modeling of a Series-Parallel Hybrid Modular Joint for Humanoid Arms

In this paper, we propose an improved design and modeling analysis of a 3-DOF series-parallel joint module for humanoid service robots. The module uses a cable-driven method to achieve motion, which is similar to human 3-DOF joints. Therefore, taking the shoulder joint as an example, it can realize the movements of arm abduction/internal retraction, back extension/forward flexion and lifting, etc. To enable the combination of modules, we design a modular connector that can be shared by different modules and allow easy disassembly. We also design a tension amplification mechanism to make the module more miniature and compact. We develop the 3-DOF transient torque model and the cross-coupling control framework, and perform the non-fixed-axis forward and inverse kinematics analysis of the 3-DOF series-parallel module based on the anti-parallelogram principle. We build the prototype and conduct load and action experiments, which demonstrate that the scheme can achieve highly realistic humanoid shoulder joint motion, providing a new design concept for humanoid service robots.